Method of making a stencil for screen-printing using a laser beam

ABSTRACT

An improved metal screen-printing stencil in which the nonprinting areas are closed with a solid polymer capable of undergoing residue-free depolymerization. The process of producing the stencil in which printing areas are opened by irradiation with a laser beam.

United States Patent Parts et a1.

[54] METHOD OF MAKING A STENCIL FOR SCREEN-PRINTING USING A LASER BEAM [72] Inventors: Leo P. Parts, Dayton; Edgar E. Hardy, Kettering, both of Ohio [73] Assignee: Monsanto Research Corporation, St.

Louis, Mo.

22 Filed: on. 6, 1969 211 Appl. No.: 864,161

[52] US. Cl. ..l0l/l28.4, 96/36.4, 117/8, 117/8.5, 117/93.31, 117/99, 204/159.14,

[51] Int. Cl ..B4lc 1/14 [58] Field of Search...346/76, 76 L, 1; 219/339, 121; 101/401.1, 395, 127, 128.4,128.2, 128.3;

[56] References Cited UNITED STATES PATENTS 2,682,687 7/1954 Franz ..264/80 2,860,576 1111958 Short ..l01/l28.3 2,924,520 2/1960 Leeds et a1 ..10l/128.3 X

3,170,792 2/1965 Cunninghamw..."10l/l28.3 X

[4 1 Oct. 10, 1972 Dombrow, B. A., Polyurethanes, Reinhold, N.Y. pages 134- 143 relied on. Piggin, B. P. Use of a Laser Beam to Remove Insulation IBM Tech. Bulletin, vol. 11, No. 7 (Dec. 1968) page 872 relied on.

Primary ExaminerRobert F. Burnett Assistant Examiner-Joseph C. Gil

Attorney-Morris L. Nielsen, L. Bruce Stevens, Jr. and Frank D. Shearin [5 7] ABSTRACT An improved metal screen-printing stencil in which the non-printing areas are closed with a solid polymer capable of undergoing residue-free depolymerization. The process of producing the stencil in which printing areas are opened by irradiation with a laser beam.

7 Claims, No Drawings METHOD OF MAKING A STENCIL FOR SCREEN- PRINTING USING A LASER BEAM BACKGROUND OF THE INVENTION open structure due to their inherently stronger and hence finer threads. Heretofore they have not realized their fullest development, particularly in preparing sharp-line stencils, because of problems in filling the screen. With gelatin, for example, it has been necessary to apply several coatings to build up a film of sufficient thickness to give fine detail in printing. Furthermore, it has been difficult to remove filler, usually organic matter, from printing areas cleanly and efficiently without partially loosening filler intended to plug nonprinting meshes in the screen. In addition, some printing detail and quality has been sacrificed by redeposition of removed filler in the fine metal screen, resulting in scummy stencils. It is well-known to those skilled in the art that screen-printing that will reproduce fine detail is much desired in preparing microcircuits, for example, by printing with resistive, conductive and dielectric inks. The present invention is directed to the solution of this problem.

SUMMARY OF THE INVENTION An object of the invention is to provide an improved metal screen-printing stencil. A further object is to provide a stencil capable of reproducing very fine detail. It is still a further object to provide an improved process for producing a metal screen-printing stencil.

These and other objects hereinafter defined are met by the invention wherein there is provided in a stencil for screen printing comprising a metal screen having printing and non-printing areas in which the non-printing areas are closed to the passage of printing ink by solid organic matter, the improvement in which the organic matter comprises a solid polymer capable of undergoing residue-free depolymerization.

As a further aspect of the invention, there is further provided a process for producing a stencil for screen printing comprising filling the apertures in a metal screen with a solid polymer capable of undergoing residue-free depolymerization, irradiating the prospective printing areas of said filled screen with a laser beam of sufficient intensity to depolymerize said polymer from said areas and completely evaporate the depolymerization products.

The silk screen process was for a long time a secret process. but now is well-known in the art (see Photomechanics and Printing, Mertle Publishing Company, Chicago, 1957, by Mertle and Monsen, Chapter 8; Silk Screen Process). Metal screens may be made of phosphor bronze, copper, molybdenum, gold, platinum or stainless steel, preferably the latter for fine detail printing. Mesh sizes of 80 to 400 may be used, the more practical being in the 200-325 mesh range.

In the present invention, in producing a stencil, the screen is coated and filled with a solid polymer capable of undergoing residue-free depolymerization. Subsequently the printing areas are exposed to the radiation of a laser so that the polymer is removed by depolymerization and evaporation of the depolymerization products, thereby leaving the screen mesh open for passage of ink.

Polymers applicable for this purpose include poly(methyl methacrylate), polyoxymethylene, polytetrafluoroethylene, poly(a-methyl styrene) and polychlorotrifluoroethylene. It is characteristic of these polymers that they are thermally degraded in the laser beam by what is believed to be a depolymerization process so that they revert to their monomeric state. Since their monomers, e.g., methyl methacrylate, formaldehyde, etc., are gases at the temperature of the polymer surface in the irradiated area no solid or molten residues remain from the degradation.

There may also be employed certain urethane formulations known in the art as solderable urethane wire enamels, see Polyurethanes, Part II Technology, Interscience Publishers, N.Y., 1964, Saunders and Frisch, pp, 580-582. These may be formulated as solutions of Mondur S, a blocked polyisocyanate hereinafter described, and either Multron R-2 or R-4, polyesters hereinafter described, applied and cured to form depolymerizable polymers. Curing conditions may be varied, as known in the art, usually employing temperatures up to 400 C. for a short period of time, e.g., 30-60 seconds at 290350 C.

We have found that these polymers admirably adhere to and fill the screen meshes and that they can be applied in thick coatings so desirable for fine detail reproduction. Upon irradiation with a laser beam whose direction and intensity is readily controlled, the depolymerized polymer leaves the screen cleanly and without residue.

The polymers may be applied to the screen as viscous solutions in suitable organic solvents, e.g. benzene, toluene, acetone, ethyl acetate, amyl acetate, etc. lt is preferable to remove the solvent prior to irradiation. The polymers may also be thermoformedto fill the screen openings, as by applying a film of the polymer to the surface and applying heat and pressure.

Instead of the preferred depolymerizable polymers named herein, there may be employed less effectively the copolymers of their respective monomers, e.g. trifluoronitrosomethane/tetrafluoroethylene; trifluoronitrosomethanel-chlorotrifluoroethylene; vinylidene fluoride/chlorotrifluoroethylene; as well as copolymers of lower alkyl methacrylates such as methyl, ethyl, butyl methacrylates; etc. There may also be employed related polymers including perfluoropropylene, poly(a,B,B-trifluorostyrene), poly(pxylylene), poly(p-tetramethyl phenylenemethylene), poly(p2,5-dimethyl phenylenemethylene), and poly(ptetramethyl phenylenemethylene). As replacements for poly(methyl methacrylate) there may be employed poly(alkyl methacrylates) where alkyl contains two to four carbon atoms.

Although the organic matter for coating the stencil of this invention must contain at least one depolymerizable polymer, it has been found that they may also contain minor proportions of other materials without adverse effect. Thus, there may be present: plasticizers, e.g. organic esters, phosphates, etc.; crosslinking agents, e.g. peroxides, or a difunctional agent up to 10 percent by weight, etc.; reinforcing agents and fillers, e.g., carbon black, fumed silica, etc.; pigments,

e.g., titanium dioxide, ferric oxide, etc.; and other materials which are readily removed from the irradiated zone as gaseous or particulate matter.

Optimum laser energy utilization and high polymerremoval speed can be attained by selecting polymers that have high absorbence at the emission wavelength of the laser. The optical characteristics of the organic matter can be altered, for the attainment of desired laser energy absorption characteristics, by incorporation of small amounts of strongly absorbing materials such as dyes. To attain desired definition and resolution, it is essential that the metal screen have high reflectivity at the emission wavelength of the laser used for the engraving of stencils. Examples of metals useful as screen materials for engraving with CO and argon lasers are: phosphor bronze, copper, molybdenum, gold, platinum, silver, and stainless steel. Useful lasers include CO argon and YAG-Nd lasers.

The laser is a device which has been well-described in the literature. It produces a high-energy, collimated beam of coherent electromagnetic radiation in the infrared, visible, and ultraviolet spectral range. Generally, lasers are classified by their type of excitation. As an example, a solid state laser, such as a ruby laser, consists essentially of a rod of the material with parallel ends polished and coated to reflect light, wherein the pumping radiation enters through the transparent sides. Another class of lasers, the gasdischarge lasers, use nonequilibrium processes in a gas discharge. The gas is excited by direct or alternating electric current, or by a radiofrequency energy source. The choice of laser may be determined not only by the power output available at its present state of development but also by its inherent wavelength output. Thus, a C laser wavelength is 10.6 um whereas a YAGzNd laser wavelength is 1.06 pm. Although the shorter wavelength can be focused to a smaller spot diameter, it may not be efficiently absorbed by many irradiated materials so that the longer 10.6 um wavelength may be preferred.

Laser emission may be pulsed or Q-switched to produce pulses, e.g. 50,000 energy bursts per second of intensely concentrated energy; or it may be continuous. Laser technology has developed rapidly, so that there are now available continuously-emitting lasers of useful power output, e.g. over 50 watts and even as high as 8,800 watts.

Since the laser produces a collimated beam that can be focused by a lens or a mirror, or deflected by a mirror, the energy flux density and direction of a laser beam can be readily controlled. Furthermore, the beam can be shaped by using stencils into which the desired patterns have been cut. For some applications, nonspherical optical lens and reflectors allow an effective means for control of the heated area. Still another way of directing the laser beam to a selected area is by backing the relatively transparent polymer-filled screen with a metallic reflector of the desired area; upon laser irradiation of the entire front surface of the filled screen at a level of intensity that would normally not degrade the polymer, the action of the beam is reinforced by the reflected beam sufficiently to cause degradation and opening of the holes.

Modulation of the laser beam is accomplished by several methods including the Kerr cell, mechanical modulators, etc. known to those skilled in the art (see Lasers, Marcel Dekker, Inc., N.Y., 1966, A. K. Levine, editor; and The Modulation of Laser Light, Scientific American, Vol. 218, No. 6, June 1968, page 17, Donald F. Nelson).

The laser engraver may be controlled by a computer, wherein the information is stored in a memory device. Alternately, a scanning system may be used in directing the laser beam in transferring information from a surface as set forth in US. Pat. No. 3,374,31 l issued Mar. 19, 1968 to R. Hell.

When a laser was investigated as' a means of opening up a stencil coated with some commercial plastics, rather disappointing results were obtained. With a phenolic composition, charring occurred so that the heated area was rough and black. With-a butyrate, yellow decomposition products deposited on the surface. With a polycarbonate, carbonaceous material formed where heated and yellow decomposition products deposited on the surface. With a polyester, the material foamed and gave a rough deposit. With a polyamide,

' melting and yellowing occurred. With polyethylene and polypropylene, there was extensive melting and flowing, with material deposited around the heated area. With polystyrene there was also material deposited around the heated area. However, surprisingly, with poly(methyl methacrylate) laser heating yielded a clean, sharply defined stencil. Subsequently, a limited number of other plastics were found to be adaptable to stencil preparation.

Previous to conducting these experiments, it was not known whether evolving vapors might undergo reactions in the vapor phase, while escaping through the path of the focused laser beam, and yield products that would deposit on the polymer surface, or whether evolving monomer vapors, while escaping through the path of the focused laser beam, might be converted to reactive molecular species that, upon reaction with the plastic stencil material would cause reduction of stencil image definition.

Cofiled applications related to this subject include Engraved Article, Ser. No. 864,162; filed Oct. 6, 1969 Engraved Rigid Polymeric Compositions, Ser. No. 864,215, now abandoned; filed Oct. 6, 1969 and Data Signal Recording Meduim, Ser. No. 864,160. filed Oct. 6, 1969.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention is further illustrated by, but not limited to the following examples.

EXAMPLE 1 A metal screen was filled with poly(methyl methacrylate). A 25 percent solution of Plexiglas (Rohm and Haas) in toluene was applied to a ZOO-mesh stainless steel screen and dried, thereby completely filling the openings in the screen.

A stencil was prepared by laser irradiation through a mask in which small slits had been inscribed. The lines were faithfully and accurately reproduced in the stencil where the polymer was clearly removed. The stencil was used for applying a conventional conductive ink Liquid Bright Gold, Engelhard) to a quartz surface. When the coated quartz was heated at 250400 C. for

30 minutes there remained sharply defined lines of fine detail.

EXAMPLE 2 A stainless steel screen having 200 meshes per inch, each with an opening of approximately 0.0029 in. (0.074 mm.) on an edge was filled with polyoxymethylene as follows. A sheet of Delrin (E. l. du Pont de Nemours and Company, Inc.) of one-sixteenth inch thickness was laid on top of the screen and the whole placed between two chrome-plated metal plates. The assembly was heated to about 380 F. (193 C.) and thereupon subjected to about 20 tons of pressure per square inch; it was cooled to 320 F. (160 C.) and the pressure released. The plastic had flowed uniformly, completely filling the openings in the screen.

A stencil was then prepared as follows: A thin brass template having the word MONSANTO excised was laid over the filled screen. The assembly was then moved across a focused CO laser beam at 0.0125 inch (0.318 mm.) intervals at a linear speed of 3.8 mm./sec. and a laser power output of watts. The laser-irradiated portions were cleanly removed of polymer leaving the word MONSANTO faithfully reproduced as an open-screen area. The stencil was useful for screen printing with conventional silk screen ink or with a metal pigmented paint.

EXAMPLE 3 opened. The whole was useful as a screen-printing stencil.

EXAMPLE 4 Solderable urethane enamels are prepared as follows, using typical formulations known in the art, e.g. Polyurethanes, Part 11 Technology, lnterscience Publishers, N.Y., 1964, Saunders and Frisch, pp. 580- 582; and Technical Information Bulletin, No. 71-C20, Urethane Finishes for the Electrical Industry, Mobay Chemical Co., Pittsburgh, Pennsylvania.

Formulation A B C D Mondur S 324.5 324.5 333.5 333.5 Multron R-2 154.5 154.5 166.5 166.5 Polyamide 24.0 24.0 Cresylic acid 204.0 347.0 207.0 500.0 Methyl glycol acetate 146.0 146.0 Butyl acetate 28.0 28.0 Toluene 119.0 119.0 High flash naphtha 150.0

Total weight 1000.0 1000.0 1000.0 1000.0 Percent total solids 50.3 50.3 50.0

All parts shown are by weight.

Mondur S is a blocked polyisocyanate adduct 6 3??llt ftlefi n ll rle r fil. iffslilis iil illg l proximately percent total solids, 1 l.5l3.5 percent available NCO, and a specific gravity equal to 1.26-1.28 at 25/25 C.

Multron R-2 is a polyester resin described by Mobay Chemical Company in their Data Sheet of November 1, 1967 as having a hydroxyl number (corrected) of 390-420, an acid number of 6.8-9.0, a specific gravity equal to approximately 1.26 at 25/155 C., and a viscosity of 800l,100 centipoises for 70 percent solids in methyl cellosolve acetate at 25 C.

The polyamide is a soluble nylon, e.g. Zytel 61, du Pont de Nemours Co., now called Elvamide 8061 as described in the du Pont Technical Information Bulletin PM l-l 165 on Elvamide Polyamide (Nylon) Resins.

Multron R-2 may be replaced by Multron R-4, using about 42 parts of Multron R-4 for each parts of Mondur S. Multron R-4 is a polyester resin described by Mobay Chemical Company in their Data Sheet of Nov. 1, 1967 as having a hydroxyl number (corrected) of 270-290, an acid number of 4.0 maximum, a specific gravity equal to approximately 1.13 at 25/155 C and viscosity of 2,000-3,000 centipoises at 73 C.

In a typical application of this invention, a solderable urethane enamel is applied to the metal screen and subsequently cured at temperatures up to 400 C., the curing conditions being governed by the physical and chemical stability of the substrate at temperature. For example a thin layer of enamel is adequately cured at 290-350 C. within 30-60 seconds. If Multron R-4 is used, lower curing temperatures are usually employed.

When the solderable urethane coating is exposed to laser radiation as described in the preceding examples, at laser outputs to 10 watts, the irradiated portions are depolymerized and cleanly removed without residue.

What we claim is:

1. A process for producing a stencil for screen printing comprising a. filling the apertures in a metal screen with a solid polymer capable of undergoing residue-free depolymerization,

b. irradiating the prospective printing areas of said filled screen with a laser beam of sufficient intensity to depolymerize and evaporate said polymer from said areas.

2. A process of claim 1 in which the solid polymer is selected from the class of poly(methyl methacrylate), polyoxymethylene, polytetrafluoroethylene, poly(amethyl styrene) and polychlorotrifluoroethylene.

3. A process of claim 1 in which the laser beam is irradiated upon selected areas of the surface by interposition of a stencil having open areas for the transmission of a laser beam.

4. A process of claim 1 in which the laser beam is ir- 

2. A process of claim 1 in which the solid polymer is selected from the class of poly(methyl methacrylate), polyoxymethylene, polytetrafluoroethylene, poly( Alpha -methyl styrene) and polychlorotrifluoroethylene.
 3. A process of claim 1 in which the laser beam is irradiated upon selected areas of the surface by interposition of a stencil having open areas for the transmission of a laser beam.
 4. A process of claim 1 in which the laser beam is irradiated upon selected areas of the surface by sweeping the surface with an intensity-modulated beam.
 5. A process of claim 1 in which the laser beam is generated by a CO2 laser.
 6. A process of claim 1 in which the laser beam is generated by an argon laser.
 7. A process of claim 1 in which the laser beam is generated by a YAG:Nd laser. 